An automotive engine uses a timing belt to drive its crankshaft, cam shafts and so on synchronously with one another. As shown in FIG. 18, a timing belt 64 is put around a crankshaft pulley 61, camshaft pulleys 62a and 62b and a drive shaft pulley 63 for an oil pump. Numerals 65 and 66 designate a tension pulley and an idler pulley, respectively. The tension pulley 65 is supported on a control arm 67 pivotally mounted on a pivot 68. An autotensioner 69 is provided in abutment with the control arm 70 to automatically adjust the pivotal motion of the tension pulley 65 and thus the tension of the timing belt 64.
This type of prior art autotensioner 69 has a cylinder case 70 filled with a hydraulic oil and a piston 71 slidably mounted in the case 70. A push rod 72 is fixed to the piston 71 so that its head will get into and out the case 70. The case has its interior separated into a pressure chamber 73 and a reservoir chamber 74 by the piston 71. A spring 75 is mounted in the pressure chamber 73 to bias the piston 71 upwardly. The spring 75 serves to bias the push rod 72 out of the case 70 while the engine is driven. The push rod 72 in turn pushes the tension pulley 65 through the control arm 67, thus preventing the timing belt 64 from loosening.
The push rod 72 is formed with a channel 76 extending from its bottom end face to side surfaces, whereas the piston 71 is formed with a channel 78 having its top open to the channel 76 and its bottom opening 77 communicating with the pressure chamber 73. A retainer 79 is secured to the bottom of the piston 71 facing the pressure chamber 73. A spring 80 has its one end secured to the retainer 79. A ball valve 81 is supported on the other end of the spring 80 and biassed by the spring 80 in such a direction as to close the opening 77, which is opened and closed as the oil pressure acting on the ball valve 81 fluctuates.
When such a force as to move the piston 71 toward the pressure chamber 73 acts on the piston while the engine is in operation, the ball valve 81 will move to close the opening 77. This will prevent any jerking movement of the piston 71 toward the pressure chamber 73, thus absorbing any fluctuation of tension of the timing belt 64 in operation. As a result, the crankshaft as well as the camshafts can be rotated in correct timing.
When the engine is stopped, some of the cams on the camshafts are in such a position as to keep their respective intake valves and exhaust valves pressed down. By the reaction force of the valve springs, the camshaft is turned in one direction until a dynamically stabilized point is reached. At this stabilized point, the timing belt will be loose at the portion between the camshaft pulleys 62a and 62b as shown for example by dotted line in FIG. 18. Instead, its tension will increase at the portion between the crankshaft pulley 61 and the camshaft pulley 62a. The increased tension of the timing belt 64 will be transmitted to the push rod 72 through a the tension pulley 65 to push it toward the pressure chamber 73. This will cause the hydraulic oil in the pressure chamber 73 to gradually flow into the reservoir chamber 74 through small gap formed around the outer periphery of the piston 71. The piston will thus gradually move toward the bottom of the case 70. In an extreme case, the piston 71 may actually hit the bottom and the timing belt will be left slack as shown by dotted lines in FIG. 18.
The timing belt tends to slacken conspicuously in a low-temperature condition because the volumes of the engine block and the cylinder head decrease. When the engine is started in such a condition, the slackness of the belt between the camshaft pulleys 62a and 62b will be removed instantly, but the belt will slacken between the crankshaft pulley 61 and the camshaft pulley 62a. The push rod 72 of the autotensioner 69 is expected to protrude and press through the control arm against the belt to remove the slackness of the belt between the crankshaft pulley 61 and the camshaft pulley 62a.
But when the engine is started in a low-temperature condition, a prior art autotensioner with the ball valve 81 may offer a problem that its push rod tends to protrude excessively to such an extent that the pressure chamber is put under negative pressure. Thus the damping action of the tension pulley 65 fails to keep constant the tension of the timing belt 64. This might cause the toothed timing belt and the pulleys to get out of meshing engagement with each other so that the belt might have its teeth damaged. This phenomenon takes place in a low-temperature atmosphere because the viscosity of the oil tends to increase sharply as the ambient temperature drops and a rather long time is required for highly viscous oil to flow from the reservoir chamber 74 into the pressure chamber 73.
Since the oil cannot flow into the pressure chamber 73 so quickly as to match the rising speed of the push rod 72 urged by the spring 75 in the pressure chamber 73, the pressure chamber will be put under negative pressure, causing cavitation. The damping action of the oil in the pressure chamber 73 will be lost completely by the cavitation. Thus, the tension pulley 65 will vibrate so violently under the fluctuating load from the timing belt 64 that the belt might have its teeth damaged).
The oil could flow through the channel, opening 77 more smoothly if the stroke of the ball valve 81 is increased. But the ball stroke has to be limited within 0.07-0.3 mm in order for the autotentioner to sufficiently dampen the fluctuating load from the timing belt 64. It is thus impossible to solve the problem by increasing the ball stroke.
It is an object of the present invention to provide an autotensioner which obviates the abovesaid shortcomings and which is free of cavitation and thus capable of driving a timing belt in a stable state.